As an evolved standard of Long Term Evolution (LTE), a Long Term Evolution Advanced (LTE-A) system supports larger system bandwidth (the largest bandwidth being 100 MHz) and is backward compatible with the existing LTE standard. In order to increase the coverage and throughput of cell edges, based on the existing LTE system, the LTE-A proposes a cooperative Multiple Input Multiple Output (MIMO) technology, also known as a Cooperation Multipoint Transmission technology (COMP), which is able to increase the spectrum utilization efficiency of an International Mobile Telecommunications-Advance (IMT-Advance) system and alleviate shortage of spectrum resources.
In wireless telecommunications, if multiple antennas are used at a transmitting terminal (e.g. node eNB), the transmission rate can be increased by spatial multiplexing, in which different data are transmitted at different antenna positions on the same time frequency resource of the transmitting terminal. Also, multiple antennas are used at a receiving terminal (e.g. a piece of User Terminal (UE)). In one case, resources of all antennas are allocated to the same user in a single user condition, i.e. in one transmission interval, one user device separately occupies all physical resources allocated to the user device, and this transmission mode is called Single-User MIMO (SU-MIMO). In another case, resources of different antenna spaces are allocated to different users in a multi-user condition, i.e. in one transmission interval, one user device and at least one other user device share physical resources allocated to these user devices, the one user device and other user devices share the same physical resource, which can be a time-frequency resource, via space division multiple access or space division multiplexing, and this transmission mode is called Multi-User MIMO (MU-MIMO).
In the LTE technology of the 3rd Generation Partnership Project (3GPP), a UE is set semi-statically via a high layer signalling to perform transmission based on one of the following transmission modes, such as:
Mode 1: single-antenna port: port 0;
Mode 2: transmit diversity;
Mode 3: open-loop spatial multiplexing;
Mode 4: closed-loop spatial multiplexing;
Mode 5: Multi-user MIMO;
Mode 6: closed-loop rank=1 precoding; and
Mode 7: single-antenna port: port 5.
The UE feeds back different channel state information to the transmitting terminal according to different transmission modes, and then the transmitting terminal (eNB) performs scheduling according to the channel state information fed back by the terminal (UE), and configures new channel state information for actual transmission according to a certain principle (e.g. a maximum capacity principle). The fed-back channel state information comprises: Channel Quality Indication (CQI) information, Precoding Matrix Indicator (PMI) information and Rank Indicator (RI) information.
The CQI is an index for measuring the quality of a downlink channel. In the 36-213 protocol, the CQI is expressed by integer values of 0 to 15 which respectively represent different CQI grades. Different CQIs are corresponding to their respective modulation and coding schemes (MCS).
The RI is used for describing the number of spatial independent channels and is corresponding to the rank of a channel response matrix. Under the open-loop spatial multiplexing mode and the closed-loop spatial multiplexing mode, the UE needs to feed back the RI information, and the RI information does not need to be fed back under other modes.
The PMI is an index number of a precoding codebook fed back by the UE. Under the closed-loop spatial multiplexing mode, the MU-MIMO mode and the R1=1 closed-loop mode, the PMI information needs to be fed back and the PMI information is not fed back under other transmission modes.
At present, the channel state information is fed back via a certain number of bits (e.g. 4 bits) in the related art, which is mainly applied to the SU-MIMO transmission mode, therefore, the fed-back information is simple with low precision. However, with the development of communication technologies, the transmission mode of dynamic switching between the SU-MIMO and the MU-MIMO is applied more and more widely in the LTE-A, and this transmission mode has higher requirements for the contents and precision of the fed-back channel state information. The original feedback method can no longer meet the requirement of the LTE-A system, especially the requirement for high precision of the channel state information under the transmission mode of dynamic switching between the SU-MIMO and the MU-MIMO.